Since the 1660Ti's are generally low power GPUs, you can try and plug in a 6 PCIe into the 8 PCIe connection on the GPU and it will most likely still work. Most of my RX 470/570 have 8 PCIe connectors but plugging in a 6 PCIe still works on them.

You can verify this with a multi-meter and see if all the GND pins are continuous and not one of them is a sensor pin. You couldn't do this with older GPUs back in the 2013 days such as the R9 280X because they had a sensor pin and made sure an actual 8 PCIe was connected but they were power hogs at > 300 Watt per GPU.

I think the reason they do this is because many people have a hard-coded 8 PCIe connector with their PSU which doesn't separate and they wanted those PSUs to get used.